1. Field of the Invention
This invention relates generally to integrated circuit assemblies, and in particular, to the electrical connection of integrated circuits to substrate circuitry, printed circuit board, and interconnect components. Most specifically, the invention relates to a flip chip assembly which includes a single or multi-layered substrate in which via holes are electrically and mechanically connected to the input/output terminal pads of the integrated circuits through direct metallization.
2. Related Art
Recent developments of semiconductor packaging suggest an increasingly critical role of the technology. New demands are coming from requirements for more leads per chip and hence smaller input/output terminal pad pitch, shrinking die and package footprints, and higher operational frequencies that generate more heat, thus requiring advanced heat dissipation designs. In addition to these demands, the more stringent electrical requirements must not be compromised by the packaging. All of these considerations must be met and, as usual, placed in addition to the cost that packaging adds to the semiconductor-manufacturing food chain.
Conventionally, there are three predominant chip-level connection technologies in use for integrated circuits, namely wire bonding, tape automated bonding (TAB) and flip chip (FC) to electrically or mechanically connect integrated circuits to leadframe or substrate circuitry. Wire bonding has been by far the most broadly applied technique in the semiconductor industry because of its maturity and cost effectiveness. However, this process can be performed only one wire bond at a time between semiconductor chip""s bonding pads and the appropriate interconnect points. Furthermore, because of the ever increasing operational frequency of the device, the length of the interconnects needs to be shorter to minimize inductive noise in power and ground, and also to minimize cross-talk between the signal leads. An example of such a method is disclosed in U.S. Pat. No. 5,397,921 to Karnezos.
Flip chip technology involves mounting of an unpackaged semiconductor chip with the active side facing down to an interconnect substrate through contact anchors such as solder, gold or organic conductive adhesive bumps. The major advantage of flip chip technology is the short interconnects which, therefore, can handle high speed or high frequency signals. There are essentially no parasitic elements such as inductance. Not only is the signal propagation delay slashed, but much of the waveform distortion is also eliminated. Flip chip also allows an area array interconnecting layout that provides more I/O than a perimeter interconnect with the same die size. Furthermore, it requires minimal mounting area and weight which results in overall cost saving since no extra packaging and less circuit board space is used. An example of such a method is disclosed in U.S. Pat. No. 5,261,593 to Casson et al.
FIG. 1 is a schematic cross-sectional view of a prior art flip chip assembly in which an integrated circuit chip 101 is attached to a substrate 102 through electrically conductive bumps 103. These bumps 103 make electrical connection between bond pads 104 formed on the chip 101 and specific one of the conductive traces 105 formed on the surface of the substrate 102. These traces 105 further extend to the other side of the substrate 103 through via holes 107 that are formed within the substrate 102. In the dielectric substrate, a via hole connects two or multiple layers of circuitry in a substrate. It can link both sides of the finished substrate, whereas a blind via link one side to one or multiple internal layers and a buried via links internal layers without being visible on the surface of the board. These via holes are typically metallized on the sidewall with copper by electroless plating and electroplating. Underfilled material 106 is typically applied between integrated circuit chip 101 and substrate 102 in order to reduce the stress due to thermal characteristic mismatch of the chip 101 and substrate 102. Conductive traces 105 formed on the top of the substrate 102 extend from the via holes to specific contacting pads or balls 108 and therefore connect to the external circuitry.
While flip chip technology has tremendous advantages over wire bonding, its cost and technical limitations are significant. First of all, flip chip technology must confront the challenge of forming protruded contact anchors or bumps to serve as electrical connections between the integrated circuit chip and substrate circuitry. A variety of bumping processes have therefore been developed. These include a vacuum deposition of an intermediate under-bump layer which serves as an adhesive and diffusion barrier. This barrier layer is composed of a film stack which can be in the structure of chromium/copper/gold. Bumping materials such as solder are subsequently deposited onto this intermediate layer through evaporation, sputtering, electroplating, solder jetting or paste printing methods followed by a reflow step to form the solder contacts.
Techniques for fabricating the intermediate under-bump barrier layer as well as the bump material utilizing electroless plating are also known. In these attempts, as shown in FIG. 2, the input/output terminal pads 201 of the integrated circuit chip 200 are firstly activated/by a catalytic solution which will selectively activate the pad material through chemical reactions and form a thin layer of catalyst 202. This thin layer of catalyst 202 is typically composed of zinc or palladium. When electroless plating is executed thereafter, material such as nickel, gold, palladium or their alloys can be selectively initiated and continuously deposited on the pads to form the bumps 203. In the above-described electroless plating process, hypophosphate or boron hydride are commonly used as the reducing agent in the nickel plating solution. This electroless plated bump not only provides the protruding contact anchor but also serves as the diffusion barrier and sealing. Contacting material such as solder, conductive adhesive or polymer is subsequently applied onto these bumps by techniques such as solder dipping, solder jetting, evaporation, screen printing or dispensing. An example of such a method is described in the U.S. Pat. No. 5,583,073, to Lin et al.
Although the electroless technique provides an economical, simple and effective method for fabricating the under bump barrier layer, contacting material such as solder or adhesive is still required for assembling. Solder dipping or screen printing of solder paste onto these bumps has been explored but with very limited success due to solder bridging and non-uniform deposition of solder on the metal bumps. This process also; it suffers from poor process control as the input/output terminal pad spacing gets smaller. Additional problems have been encountered with tin/lead solder due to its increase in electrical resistance over time. Moreover, the solder contacts are easily fatigued by mechanical stressing.
Organic contacts which utilize conductive adhesive to replace solder joint is also described by U.S. Pat. No. 5,627,405, to Chillara. Generally speaking, the conductive adhesive, which is made by adding conductive fillers to polymer binders, holds a number of technical advantages over soldering such as environmental compatibility, lower-temperature processing capability, fine pitch and simplified processes. However, conducive adhesive does not normally form the metallurgical interface in the classical sense. The basic electrical pathway is through conductive particles of the adhesive that are in contact with one another and reach out to the two contact surfaces of the components. Under certain environments, this interconnect system may cause problems because the penetration of moisture through the polymer may induce corrosion and oxidation of the conducting metal particles which results in unstable electrical contacts. Furthermore, failure of the joints can also occur due to degradation of the polymer matrix as well as degradation of the metal parts. Since the electrical and mechanical performance are independent of each other, good mechanical performance is no assurance of electrical integrity.
In view of the limitations in the currently available integrated circuit assembling methods, a high-performance, reliable and economical method which interconnects integrated circuits to the external circuitry would be greatly desirable.
According to the invention, a flip chip assembly is provided to address high density, low cost and high performance requirements of electronics products. It involves the direct interconnection of integrated circuit chip to substrate circuitry through direct metallization of via holes and bond pads without the need for bumps, wire bonds, or other media.
To achieve the foregoing and in accordance with the invention, the assembly includes a rigid or flexible dielectric substrate having a plurality of electrically conductive circuitry, traces one or more integrated circuit chips having a plurality of input/output terminal pads, and a plurality of via holes formed in the dielectric substrate for electrically connecting respective traces of the substrate with respective pads of the integrated circuit chip. The surface of the integrated circuit chip and the dielectric substrate may be arranged in substantially mutually parallel planes. The orientation of the contact is in such a manner that the via holes in the dielectric substrate are aligned with the top of the pads of the integrated circuit chip so that these pads can be totally or partially exposed through the opposite side of the substrate. After the alignment, the connecting step may include attaching the integrated circuit chip to the dielectric substrate through mechanical or chemical techniques to form an assembly. Electrically conductive material is subsequently deposited in the via holes as well as on the surface of the input/output terminal pads of the integrated circuit chip to provide electrical and mechanical connections between the terminal pads and the traces of the dielectric circuitry. After the via holes are connected to the terminal pads, the mechanical and chemical means that provided the chip and substrate attachment can be removed or left as an integral part of the assembly since the via hole connections also provide mechanical support.
In a method aspect of the invention, the connection is provided by electroless plating. The electroless plating initiates and continuously deposits electrically conductive material such as copper, nickel, palladium, gold and their alloys on the via hole side walls as well as input/output terminal pads of the integrated chip. As the plating process continues, the metallic surface of the via holes sidewalls and terminal pads will extend out and contact each other and finally join together and become an integrated part. These simultaneously electrolessly plated joints provide effective electrical and mechanical connections between the integrated circuit chip and the dielectric circuitry.
In another method aspect of the invention, the connection may take the form of electrochemical plating. In this method, metallized via holes in the dielectric substrate are electrically connected to an external power source and serve as one electrode for plating. This plating process can be carried out on the sidewalls of via holes as well as other areas that receive electricity from the power source and are exposed to the electrochemical plating solution. In the initial stage, the terminal pads of the integrated circuit chip do not receive electroplating due to lack of electrical contact with the power source. However, as the via hole sidewall plating process continues, the metallizing surface will extend out and finally contact and provide electricity to the terminal pads and subsequently initiate electroplating on them. These simultaneously electroplated parts join together and provide effective electrical and mechanical connections between the chip and the dielectric circuitry.
According to a further aspect of the invention, the connection may take the form of solder paste, liquid solder, solder particles, epoxy or conductive polymer which is reflowable and bondable to the integrated circuit chip terminal pads and via hole walls after the application of an energy source such as a laser or infrared light. In this method, the filling material can be filled into the via holes through selective printing, jetting or ball placement techniques. As the external energy such as heat or a laser applied to the filling material, the original form of the material will melt and change its shape, enlarge the contacting areas, and adhere to the wettable surfaces thus providing an effective electrical and mechanical contacts between pre-metallized via hole sidewalls and input/output terminal pads of the integrated circuit chips. In some embodiments, these input/output terminal pads are pre-treated or coated with a thin protective layer if the material is vulnerable to corrosion or dissolution through reactions by the joint material such as solder.
According to the invention, via holes of the substrate can be formed by various techniques including mechanical drilling, punching, plasma etching or laser drilling. They are formed in the substrate at locations where electrical circuitry on one side of the substrate can be connected to the opposite side of the surface on which the semiconductor chip or chips are mounted and their input/output terminal pads can be exposed through these holes.
According to the invention, dielectric layers of the rigid substrate can be either organic or inorganic material. An organic type substrate is preferable for lower cost, and superior dielectric property whereas an inorganic substrate is preferable when high thermal dissipation and matched coefficient of expansion are desired. Suitable dielectrics include plastics, ceramics and flexible films.
If the finished product is, for instance, a ball grid array, solder balls can be formed on the traces. This finished package can be connected to a printed circuit board by reflowing the solder balls to form an attachment to the conductors on the surface of the printed circuit board.
In summary, using via hole direct connection of an integrated circuit chip and dielectric substrate circuitry instead of anchoring solder or a conductive adhesive bump allows a high reliability, low profile, and high performance assembly to be achieved. In particular, a small via hole formed by laser drilling or other techniques allows a very fine pitch terminal pad to be interconnected, which can significantly enhance the capability of packaging future high I/O semiconductor chips.